Accelerometers can be used to detect engine knock. Engine knock can be caused by pre-mature and uncontrolled detonation of the charge inside the combustion chamber, which can be caused, for example if an Otto-Cycle engine has been fueled with a fuel with too low an octane rating, or if spark timing is too advanced, or if deposits in a combustion chamber create hot spots that cause early ignition. In a Diesel-Cycle engine, engine knock can be caused, for example, if fuel injection timing is too early. Engine knock can cause a decrease in engine performance and in severe cases, if not corrected, engine knock can cause serious damage to the engine, including destruction of the pistons, connecting rods, exhaust valves, head gasket and spark plugs or glow plugs. Accelerometers that are employed as “knock sensors” are typically located on an engine's cylinder block and sometimes on the cylinder head. Engine knock correlates to violent combustion events that are not part of normal combustion behavior. Accordingly, engine knock is not a characteristic of combustion behavior that is normally measured in each combustion cycle. Most knock sensors send a base or “no knocking” reference signal to the electronic engine controller and an easily detectable higher signal when engine knock is detected. Because there is a relatively large difference between the reference signal and the signal when engine knock is detected the accuracy of the knock sensor is relatively unimportant. In this respect, knock sensors are specialized to detect only engine knock.
A signal that is characteristic of normal combustion behavior in a combustion cycle contains more information and needs to be much more accurate than a signal that is normally needed from a knock sensor. Beyond guarding against severe engine damage that might be caused by engine knock, more accurate combustion behavior information can be used to better control or diagnose combustion in an internal combustion engine to improve engine performance and efficiency, and/or engine-out emissions. The operation of most types of internal combustion engines can be improved if an engine controller is provided with accurate information about combustion behavior, such as, for example, information about combustion phasing, which includes the timing for the start of combustion (“SOC”), the combustion rate, which includes the heat release rate as one indication of the combustion rate, the in-cylinder pressure, and engine misfiring. With accurate combustion behavior information such as this, engine performance can be improved by adjusting parameters such as, for example, the timing for fuel injection, the fuel injection rate, and the quantity of fuel injected. Furthermore, a particularly useful application for this type of combustion behavior information is what is known as a homogeneous-charge compression-ignition (“HCCI”) engine. Although HCCI engines have not yet been widely commercialized, in recent years significant work has been directed to developing such engines because they offer the potential for higher efficiency and lower engine-out emissions compared to conventional compression ignition Diesel-Cycle engines and spark-ignited Otto-Cycle gasoline engines. For example, at highway cruising conditions the Diesel-Cycle engines in Class 8 heavy-duty trucks operate at about 40% brake thermal efficiency, and these heavy trucks typically achieve only about 6 miles per gallon. HCCI engines could improve engine efficiency significantly, giving a gain in fuel economy. For vehicles that use gasoline-based engines, which are even less efficient than their diesel counterparts, the potential fuel savings is greater still. In addition, because of the potentially lower engine-out emissions from HCCI combustion, HCCI engines might allow future diesel-fueled engines to avoid selective catalytic reduction and its complicated and expensive system of multiple catalysts.
One of the main challenges delaying the commercial introduction of HCCI engines is the difficulty in controlling HCCI combustion and a system that generates an accurate signal characteristic of combustion behavior can be used to solve this challenge. For example, a production-ready SOC sensing system could help to enable HCCI combustion over a wide range of conditions. To be successful, such a sensing system should meet the important practical needs of commercial automotive products, namely low cost, reliability and durability.